A conventional light emitting diode (“LED”) device uses an epoxy polymer resin as encapsulating material. The encapsulation process is frequently accomplished by injection molding, transfer molding or casting. Cured epoxy encapsulant has relatively high hardness, which provides resistance to scratches and abrasion, high rigidity, and high initial light transmissivity. Conventional encapsulated LED devices come in a variety of sizes and styles, such as 4 mm oval LED Lamps, 5 mm round LED Lamps, chip LEDs and plastic leaded chip carriers (“PLCCs”).
However epoxy-based encapsulating materials suffer from thermal- and photo-degradation. Degradation is especially acute if the wavelength emitted by the LED die is in the near ultraviolet (“UV”) or UV portion(s) of the spectrum. Epoxy encapsulating material degrades when subjected to high light flux, particularly if the wavelength of the light is in the range from 200 nm to 570 nm. Degradation of the encapsulant results in increased absorption of light in the blue to green wavelengths, causing a “yellowing” effect on clear epoxy encapsulant and reduced light transmissivity through the encapsulant, which causes a significant drop in the light output of the LED device. Typically, an epoxy-based 5 mm LED lamp device's light output drops by 20% or more after 1000 hours in use, and by 50% or more after 10,000 hours in use.
Another problem with conventional epoxy-encapsulated LEDs can arise if the LED is driven with a current greater than 100 mA because heat generated by the LED during operation can degrade the epoxy. Furthermore, epoxy-based encapsulant is hard and stresses wire bonds in LED devices during temperature cycling, which can lead to premature failure.
FIG. 1 shows a conventional LED lamp 100. An LED die 102 is attached to a substrate portion 104 of a first lead 106. The backside of the LED die 102 is a first LED die terminal. A second LED die terminal 108 is connected to a second lead 110 with a bond wire 112. In a particular embodiment, the LED die is mounted in a reflector cup (not shown) of the substrate portion using conductive epoxy. The bond wire 112, LED die 102, substrate portion 104, and portions of the leads 106, 110 are encapsulated in hard, rigid epoxy encapsulant 114. The hard, rigid epoxy encapsulant 114 protects the bond wire 112 from being damaged when the LED lamp is handled during manufacturing, transport, and assembly into a circuit.
Silicone polymer materials are less prone to yellowing and thermal degradation than the epoxy used in conventional LED lamps. Silicone materials are also relatively soft, and therefore less likely to cause thermal cycling failures. However, since silicone is soft, it does not form a durable, protective, hard coating for an LED die and associated bond wire(s). Therefore, LED lamps fabricated with soft, high-temperature, non-yellowing encapsulants, yet providing a hard, durable outer cover, are desirable.